the "Curiosity mission clock starts from mean local solar midnight immediately preceding touchdown, and for the midnight at the originally planned landing longitude and not the actual landing longitude. As MSL touched down mid afternoon local time, a clock started at the time of touchdown would lag between the mission clock by about 15 Mars hours." [use SpaceCraft Event Time (SCET) rather than Earth Received Time (ERT) which may be "off by about 14 minutes"].

in summary, "if you plug the time and date UT 2012-08-05 13:50:00 into an MSL timekeeping calculation, you should obtain that it is mission time 00:00 on Sol 0. If you're off by a second or two, you're doing as well as Mars24. If you're off by about 7-8 seconds, you may be using the landing longitude in the calculation rather than the planned longitude. If you're off by about 14 minutes, then you probably have ERT rather than SCET."